1. Field of the Invention
The present invention relates to a gear having teeth on the outer surface thereof, a reduction gear combination using the gear as a pinion and/or a gear wheel, and an electric power steering apparatus comprising the reduction gear combination.
2. Description of Related Art
FIG. 1 is a sectional view showing the structure of a conventional reduction gear combination provided in an electric power steering apparatus.
The reduction gear combination provided in an electric power steering apparatus comprises a worm 102 having one end connected to a drive shaft 101 of a steering assist motor 100 and a worm wheel 103 engaged with the worm 102. The worm wheel 103 is fixed on a rotary shaft 104. The rotary shaft 104 is supported in a housing 105 by a couple of antifriction bearings (which are not illustrated in the figure).
The worm 102 includes a right cylinder member made of metal and a helical tooth which is integrally formed on the outer surface of the right cylinder member. The worm 102 is supported in the housing 105 by a couple of antifriction bearings 106 and 107, with the axis thereof crossing the axis of the rotary shaft 104.
The worm 102 and worm wheel 103 of the reduction gear combination are assembled in such a manner that movement in the radial direction of each gear is prevented. However, since a dimensional error occurs in each of manufactured worms 102 and worm wheels 103, the backlash amount at the engagement portion of the worm 102 and worm wheel 103 varies if the gears are assembled taking no account of the dimensional error. When the backlash is large, rattling noise occurs during steering and heard in the passenger's room of a car. On the other hand, when the backlash is small, a load is increased and the worm 102 and worm wheel 103 cannot be rotated smoothly.
To solve this problem, in the prior art, a stratified assembling method is employed to set a proper backlash amount at the engagement portion. In this method, manufactured worms 102, worm wheels 103, antifriction bearings 106 and 107, housings 105 and the like are first sorted into a plurality of dimension groups for each design dimension. A set of a worm 102, worm wheel 103, antifriction bearings 106 and 107 and housing 105 to be assembled is then selected from the sorted dimension groups, so that the centre distance H between the worm 102 and the worm wheel 103 is set within a permissible range.
However, in the above method for manufacturing a reduction gear combination and an electric power steering apparatus wherein a worm 102, worm wheel 103, antifriction bearings 106 and 107 and housing 105 to be assembled are selected from a plurality of sorted dimension groups, strict dimension control is required and the amount of inventories to be selected from is large. Consequently, there arises a problem that assembling needs much time, which causes high cost.